Despite an expanding role of stereotactic radiosurgery, a number of key questions remain unanswered regarding the fundamental radiobiological and histopathological mechanisms contributing to radiosurgical obliteration of brain arteriovenous malformations (AVMs). Radiosurgical dose selection remains largely empirical, and AVM characteristics (volume, hemodynamics, vessel caliber) that may dictate dose dependent radiosurgical occlusion have not been studied adequately. The long-term objective of this project is to improve the overall clinical management of patients harboring brain AVMs by increasing the efficacy and safety of stereotactic radiosurgery. This may be achieved by a more thorough understanding of the underlying radiobiological principles, obtained through investigations in an appropriate experimental model. The immediate objective of this project is to use a reliable in vivo model of experimental AVM developed in our laboratories for radiosurgical research: 1) To study characteristics of AVM that may affect its response to radiosurgey. In particular we will investigate the role of AVM hemodynamics as a variable in altering the response of the lesion to radiosurgery. 2) To perform detailed histopathological studies of radiosurgical effects and investigate how this leads to vessel occlusion. 3) To elucidate the molecular factors responsible for the morphological changes observed in an AVM following radiosurgery. In particular immunohistochemical analysis of irradiated tissue will be performed looking for the presence of cytokines that may have a role in this process. Through these efforts, we hope to enhance the success and safety of stereotactic radiosurgery for patients with brain AVMs.